Ferritic stainless steels containing no nickel as an alloying element have advantages that their production cost is low and they are free from various types of stress corrosion cracking, but on the other hand have disadvantages that general corrosion resistance is poor and their weldability, corrosion resistance and mechanical properties in welded portions are poor so that they have been restricted in their wide application, particularly their application in the fields such as chemical plants where high degree of material reliability is required. Presently, austenitic stainless steels have been mostly used in these fields, but they have a defect of the susceptibility to stress corrosion cracking so that their reliability in these fields is not enough.
In recent years, several new grades of austenitic stainless steels have been proposed as stress corrosion resistant steels, but most of them do not always show reliable resistance to stress corrosion cracking in actual services although they are immune to cracking in boiling 42% MgCl.sub.2 solution.
This is due to the fact that the laboratory test condition in boiling 42% MgCl.sub.2 solution is not identical to the actual condition in service. Therefore, great cares have been required in selection of the test condition for development of new corrosion resistant steels, particularly new corrosion resistant steels having satisfactory resistance to stress corrosion cracking. If such cares are not taken, the resultant steels have not practical utility.
The present inventors have conducted studies and development works taking these considerations and have succeeded in development of a novel ferritic stainless steel which is free from any type of stress corrosion cracking, having similar or better corrosion resistance than that of an austenitic stainless steel, and which has eliminated poor weldability and deterioration of corrosion resistance and mechanical properties in welded portions with which the conventional ferritic stainless steels confront inherently.